In the medical field, there are numerous applications for pumping mechanisms, that generally operate to pump liquid and/or compressible gas mixtures by repeatedly squeezing a flexible tube to push the pumped substance therethrough. Two examples of such pumping mechanisms are "roller pump" peristaltic pumps and linear peristaltic pumps.
Typically, "roller pumps" employ a stator having a bearing surface against which one or more hoses is compressed by a rotating rotor, the rotor engaging the hoses with two or more rollers. On rotation of the rotor, the fluid in the hose or hoses is transported in the direction of the rotor's rotation. Alternatively, the fluid can be presented to the pump under pressure, such that rotation of the rotor causes the pump to serve as a measuring valve. In either instance, knowledge as to the inner diameter of the hose or hoses and the rotational speed of the rotor provides a knowledge of the amount of fluid passed through the hose or hoses, which amount can be regulated by regulating the speed of the rotor.
Examples of prior art "roller pump" peristaltic pumps, used for drug administration devices, are proposed in U.S. Pat. No. 4,576,556 to Thompson, entitled "Roller Pump," and in U.S. Pat. No. 4,692,147 to Duggan, entitled "Drug Administration Device." The Thompson '556 and Duggan '147 patents are commonly assigned to the assignee of the present invention and are hereby incorporated by reference herein in their respective entireties. It has been demonstrated that peristaltic pumps such as those described in the Thompson '556 and Duggan '147 patents provide a highly reliable mechanism for inclusion in a totally body-implantable drug infusion pump including a control system, power source, fluid reservoir, and refilling mechanism.
Linear peristaltic pumps typically have a series of fingers or cams that contact and compress a flexible tube in a sequential linear fashion so that fluid in the tube is pushed along ahead of the closing fingers or cams. Examples of linear peristaltic pumps include U.S. Pat. No. 4,482,347 for "Peristaltic Fluid-Pumping Apparatus" issued to Alexander S. Borsanyi on Nov. 13, 1984, U.S. Pat. No. 4,909,710 for "Linear Peristaltic Pump" issued to David E. Kaplan, David Burkett and Laurence Warden on Mar. 20, 1990 and U.S. Pat. No. 5,217,355 for "Two-Cycle Peristaltic Pump with Occlusion Detector" issued to Oscar E. Hyman, Ahmadmahir M. Moubayd and Larry L. Wilson on Jun. 8, 1993
A desirable characteristic of either type of peristaltic pump is the fact that the substance being pumped does not come into contact with any component of the pump other than the inside of the flexible tubing; thus, sterility of the substance is preserved. Peristaltic pumps are also known to be volumetrically accurate and to have a high degree of constancy and uniformity of flow. In addition, the positive displacement nature of peristaltic pumping mechanisms render them capable of pumping fluid/compressible gas mixtures and fluids with varying viscosities. Peristaltic pumps have proven to be highly reliable for application with, among others, aqueous drug formulations.
Prior art peristaltic pumping mechanisms for drug delivery or infusion typically use an elastomeric tubing circuit as the conduit for moving the drug through the pumping circuit by displacement during tubing compression by peristaltic pumping rollers. Potential disadvantages of prior art designs using elastomeric tubing include the inability to prevent permeation of water vapor and drug components from the drug formulation through the tubing wall, potentially resulting in adverse corrosive effects on the pump motor and gear drive train assembly.
Permeation of water vapor and drug components through the peristaltic tubing wall can also lead to reduced product longevity due to the increased mechanical friction or compromised electrical isolation with increased parasitic electrical resistance in excess of motor output capacity. Moreover, certain drug formulations may require the use of lipophilic agents or organic solvents due to low aqueous solubility. Such agents or solvents may not be compatible with the elastomeric peristaltic tubing, and elastomer degradation or swelling may arise, causing compromised product life.
Parasitic mechanical friction produced by passing the roller over a drug-swollen elastomeric surface creates an additional energy requirement, which can further limit the pump's functional longevity.
In the context of a fully implantable device, exemplified by the SynchroMed.RTM. drug infusion system commercially available from Medtronic, Inc., Minneapolis, Minn., the foregoing and other considerations are of particular concern. If the elastomeric pump circuit does not establish a hermetic barrier to water vapor permeation, the reduction gear drive mechanism, motor coil, and electronic circuitry will be exposed to a humidified environment. With prolonged exposure to such a humidified environment, gear lubricant breakdown can occur, leading to increased gear wear and friction to moving parts. Secondarily, certain elastomeric tubing, such as silicone polymers and the like, are prone to absorption of various components of the therapeutic formations being pumped. This absorption results in elastomeric swell which manifests itself in either reduced life due to friction or immediate pump stall due to frictional forces in excess of pump driving force.